Covalent Bonds
Section: 2. Atoms, Elements & Compounds | Syllabus: Cambridge IGCSE Chemistry (0620)
What is a Covalent Bond?
A covalent bond forms when a pair of electrons is shared between two atoms, so that both atoms gain a stable, noble-gas electronic configuration. Covalent bonding occurs between non-metal atoms, since neither atom can easily give up electrons to the other - sharing lets both fill their outer shells at once.
Shared electron pairs sit in the overlap between two atoms and count towards both atoms' outer shells at once
Core Examples: H₂, Cl₂, H₂O, CH₄, NH₃, HCl
| Molecule | Bonds formed | Structure |
|---|---|---|
| Hydrogen, H₂ | 1 single bond (H shares its 1 electron) | H-H |
| Chlorine, Cl₂ | 1 single bond (each Cl shares 1 of its 7 outer electrons) | Cl-Cl |
| Water, H₂O | O forms 2 single bonds, one to each H | H-O-H |
| Methane, CH₄ | C forms 4 single bonds, one to each H | H around a central C (tetrahedral) |
| Ammonia, NH₃ | N forms 3 single bonds, one to each H | H-N-H with a third H and a lone pair on N |
| Hydrogen chloride, HCl | 1 single bond | H-Cl |
Each element tends to form a fixed number of bonds, based on how many more electrons it needs to complete its outer shell: H = 1 bond, O = 2 bonds, N = 3 bonds, C = 4 bonds, Cl = 1 bond.
Worked Example: Methane, CH₄
- Question: carbon has the electronic configuration 2.4. Explain how carbon forms covalent bonds with four hydrogen atoms in methane, CH₄. [2]
- Answer: carbon has 4 outer electrons and needs 4 more to reach a stable configuration of 8, so it shares one electron with each of 4 hydrogen atoms, forming 4 single covalent bonds [1]; each hydrogen shares its 1 electron with carbon, giving each hydrogen a full outer shell of 2 [1]
Supplement Examples: O₂, CO₂, N₂, CH₃OH, C₂H₄
A double bond is 2 shared pairs of electrons (=); a triple bond is 3 shared pairs (≡).
- Oxygen, O₂: each O atom (6 outer electrons) needs 2 more, so the two atoms share 2 pairs → O=O (double bond)
- Carbon dioxide, CO₂: carbon forms a double bond to each oxygen → O=C=O
- Nitrogen, N₂: each N atom (5 outer electrons) needs 3 more, so the two atoms share 3 pairs → N≡N (triple bond)
- Methanol, CH₃OH: carbon forms single bonds to 3 hydrogens and to the oxygen; the oxygen forms a second single bond to the remaining hydrogen (H₃C-O-H)
- Ethene, C₂H₄: the two carbon atoms share a double bond (C=C), and each carbon also forms a single bond to 2 hydrogen atoms
In ethene the two shared pairs of the C=C double bond count towards both carbons' outer shells, alongside their single C-H bonds
Common Mistakes
- ❌ Thinking a double bond means two entirely separate single bonds between different atoms
- ✓ a double bond is 2 shared pairs of electrons between the same two atoms, not two bonds to two different atoms
- ❌ Forgetting that a shared pair of electrons counts towards both atoms' outer shells at the same time
- ✓ each electron pair in a covalent bond is counted twice - once for each atom sharing it - when checking that an atom has a full outer shell
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